Final answer:
Thoroughbred racing in the early United States failed to gain popularity due to prohibitions on horse races and gambling by the delegates. Exclusionary rules that limited participation of African Americans also contributed to the lack of popularity of the sport.
Step-by-step explanation:
Thoroughbred racing in the early United States failed to gain popularity due to several factors. Firstly, the delegates of the time period prohibited horse races and other similar events because they encouraged gambling, which was seen as a threat to unity and moral values. Additionally, the sport of horse racing eventually created rules that excluded African Americans, leading to a lack of diversity and exclusion of potential talent.